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Some tropical plants, including some houseplants like the (iron plant aspidistra), will survive under light as dim as 50 foot candles. The flowering reaction of other plants is even more sensitive. As little as one-foot candle can change a plant’s reaction and throw it into flowering, or prevent this response, depending on whether it is a long or short day plant.

There are yet other plants that need 7,000 or more foot candles and refuse to flower unless exposed to full sun. I would assume that if you could provide the full light spectrum – blue light, red light and everything in between. However, I will say that I’ve seen some amazing picture perfect African violets grown under artificial lighting indoors. Oddly enough, warm-weather plants can stand lower light intensities at higher temperature than cool-weather plants can. Plants like cinerarias and calendulas needing relatively low temperatures will suffer more in a dark room at high temperatures than will the poinsettia, unless they get extra light. One of the weaknesses of the use of artificial lighting in growing plants has been the high heat generated by incandescent light bulbs. With fluorescent grow light bulbs, much of this handicap has been removed.

Fluorescent tubes are capable of giving off a high output of light with less heat. None the less, the three “dimensions” of light: … present a problem. Light intensity refers to the strength of the light, usually measured in foot candles. Photographic light meters are frequently rated in foot candles, so this term is not as esoteric as it once was. The duration refers to the number of hours the light source sheds its radiation, but duration and intensity are not interchangeable. You cannot make up for a short day by making the light brighter for fewer hours. There is some evidence that African violets can be grown as well with 600 foot candles for 18 hours as they can with 1,000 foot candles for 12 hours, but this is about the only case where the two qualities are not interchangeable. Full-spectrum fluorescent light bulbs can, of course, be burned as long as is necessary. Because the radiation from these full-spectrum light bulbs or fluorescent tubes does not “project” well, plants must be placed within 12″ inches of the lighting fixture to get enough “grow light.” For this reason, a standard type of light fixtures equipped with two 40-watt fluorescent lighting tubes will only light about 4 square foot of growing space – about 12″ inches wide and four feet long. Incandescent bulbs, on the other hand, can be suspended three feet above the plants. When used in this way, one 60-watt bulb will substitute for the sunlight on an area four feet by four feet. However, this statement should not be interpreted to mean that either fluorescent lights or incandescent bulbs can be used to substitute for natural light from the sun on a full-time basis. For many species, such lighting can be used as a supplemental light source. If the plant is one which reaching the flowering stage during the shorter days of the year, such extra light can help to hold it back. If it is one that needs long days for normal blooming, the extra lighting may make the difference between flowers and no flowers. In the case of the poinsettia, for example, the usual failure of this plant when grown in the ordinary living room can be traced to the fact that the room is used at night. Sun lamps for plants equal to just two or three foot candles is enough to interrupt flowering. When a family member reads their magazine, iPad or Kindle with the living-room lamp on, a poinsettia cannot be expected to flower in that room. It needs complete darkness for at least 13 to 14 hours a night before it will flower: The chrysanthemum, also a short-day plant, is often controlled by supplying it with extra light until late in Winter. When flowers are wanted, the lights are turned off, and the plant flowers.

Thus chrysanthemums can, by shading in Summer and lighting in Winter, be made to flower at the convenience of the grower. Comparing Types Of Artificial Light On Plant Growth. The amateur grower can use an artificial sunlight lamp for several purposes. One of the most interesting is to force tuberous begonias into bloom during the dark days of Winter. Being a long-day plant, the tuberous begonia needs extra light. In the open, it stops forming flower buds when the length of the day drops below 12 hours. About mid-September, over the entire United States, tuberous begonias quit flowering and form tubers instead. At Cornell University, they found that, if lights are used on the plants just before this condition happens, starting in mid-August, the tubers do not increase in size, and the plants keep on flowering all Winter long. Either fluorescents or incandescents lighting systems can be used as light for indoor plants, by turning them on at four in the afternoon and off at nine at night. A great many houseplants such as geraniums, Browallia, Heavenly Blue morning glories and other plants can be kept in bloom throughout the Winter by this same supplemental lighting.

Using artificial sunlight for plants source, foliage plants needing bright light like coleus will keep short and sturdy. English ivy will stop reaching for the sky, and remain short-jointed. This practice can hardly be considered as growing plants under completely artificial light.

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